It is possible to change acceleration and top speed of a car, you would need to extract their files from misc_tables.vpp_pc. They usually have a prefix, like so:
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Open each file to find out what car it is, under <DISPLAY_NAME>:
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To edit acceleration, simply changing<Torque> will do.
You can also edit other tags such as <Wheel_Engine_Torque> to tweak . However, all cars in this game stick to the ground like glue anyway so this is only noticeable if you handbrake or change <Viscosity_Friction>.
Top speed is helpfully labeled as <Top_Speed>. This is a "hard cap", the car will never go above this value no matter the torque.
The game seems to cap Top Speed to 100 globally and I have no idea how to change it. I know that trainers like ZolikaTrainer for original game can do it though... If anyone knows what file to change this I'd be eternally grateful.
Note that like any other mod, you need to put the .xtbl files in same folder as game .exe which gets quite messy.
The concept of gears doesn't exist in this series, all cars accelerate from 0 to top speed without any power loss/torque curve changes. The illusion of having gears is just audio; if you're on a highway you can easily notice the car "upshifting" despite staying on the same top speed.
If you want to tweak it anyway, see the RPM tags under <Engine> and <Transmission>. I have no clue how gear ratios work though.
Final note, I recommend using Notepad++ to search for the tags easier.
Hope it helps!